Cross-polarization interference, that is, interference due to signals whose polarization is supposed to be orthogonal to the polarization of the receiver, has created problems in terrestrial and satellite microwave communication systems. In, for example, satellite communication systems, larger earth stations include adaptive techniques for improving cross-polarization isolation. These techniques tend to be systems which mechanically rotate the antenna feed to compensate for changing conditions. In this regard see, for example, the article "A Dual-polarized 4/6 GHz Adaptive Polarization Control Network" by A. E. Williams in Comsat Technical Review, Vol. 7, No. 1, Spring 1977 at pp. 247-262.
Another arrangement is disclosed in U.S. Pat. No. 3,883,872 issued to J. C. Fletcher et al. on May 13, 1975 which relates to a receiving system for automatically selecting a desired one of two approximately orthogonally polarized signals occupying the same bandwidth. The received signals are provided by any orthomode antenna system at a pair of output ports and then applied to the inputs of a hybrid junction to produce sum and difference signals. The resulting sum signal at one output port comprises components of the undesired one of two orthogonally polarized signals and is used to coherently detect and dynamically balance out the undesired signal components that are included at the difference signal port. The desired one of two orthogonally polarized signals is thereby provided at the difference port of the hybrid junction, and feedback loops are used to effect dynamic balancing.
Still another arrangement is disclosed in U.S. Pat. No. 3,943,517 issued to G. F. Vogt on Mar. 9, 1976 which relates to an adaptive polarization receiving system wherein a polarization follower is employed to make the system phase angle track the orientation angle of linear polarization of the received carrier so that the receiver apparatus will follow polarization changes in the information signal. This apparatus includes a closed loop feedback network in which an error signal is generated whenever the system phase angle differs from the incident polarization angle, and in which a control voltage is developed for superimposition on a signal which scans the receiving antenna at a constant frequency. At the same time, the error voltage is used to optimize the reproduction of the received message information in the presence of variations in incident polarization angle by off-setting the signal fading which results.
Adaptive co-channel interference suppression systems have also been designed but do not solve the problem of cross-polarization interference. In this regard see, for example, "An Adaptive Co-channel Interference Suppression System to Suppress High Level Interference in Satellite Communication Earth Terminals" by E. D. Horton in National Telecommunication Conference Record, Dallas, Tex., Nov. 29-Dec. 1, 1976, Sect. 13.4, pp. 1-5 and "Suppression of Co-channel Interference with Adaptive Cancellation Devices at Communications Satellite Earth Stations" by P. D. Lubell et al in ICC 77 Conference Record, June 12-15, 1977, Chicago, Ill., Vol. 3, pp. 49.3-284-49.3-289. In these disclosed systems, an independent sample of the interfering signal is obtained, the phase and amplitude of which is adjusted by an adaptive filter to provide an estimate of the interference in the received signal. This estimate is then subtracted from the received signal to give the undistorted desired signal and a residue from the subtract operation. The response of the adaptive filter depends on the correlation between this residue and the interference sample.
Another co-channel interference suppression arrangement is disclosed in a co-pending patent application Ser. No. 81,552 filed on the same day as the present application for D. M. Brady and A. M. Gupta wherein a main antenna picks up the desired signal and some interfering signal and a small auxiliary antenna is pointed in the direction of the interfering source and picks up a sample of the interfering signal. The interfering sample is then put through a quadrature modulator for adjustment of its phase and amplitude and for providing an estimated cancellation signal at the output. This estimated cancellation signal is then combined with the main antenna output to give a corrected signal. After down-converting, the present system detects the power in the corrected signal and a processor in response to such power detection generates a small dither signal which is added to the control signals to vary the phase and amplitude in the quadrature modulator of the residual interference in the corrected signal.
The problem remaining in the prior art is to provide an adaptive cross-polarization cancellation arrangement which is both simple and inexpensive and provides excellent isolation and response to changing conditions in comparison with known arrangements.